The prostatic acid phosphatase (PAP) is a promising immunological marker enzyme for prostate carcinoma cells. The objectives of our investigations are to define the antigenic specificity of PAP in relation to acid phosphatases of other tissue origin and to develop immunoassay methods for PAP to aid the diagnosis of prostate cancer. We have introduced innovations in the PAP purification procedure which is suitable for immunochemical studies of this enzyme. We have established the immunochemical identity of electrophoretic (or chromatographic) isozymic forms of the PAP. Antigenic specificity of the PAP and the kinetics of PAP-anti-PAP antibody interactions were studied in detail. We have developed counterimmunoelectrophoretic assay and radioimmunoassay for the PAP in the biological fluids and immunofluorescence technique for PAP secreting cells. These techniques were applied to the determination of serum and bone marrow PAP levels and to the identification of prostate carcinoma cells metastasized to the lymph nodes and bone marrow. Further studies on the antigenic structure of PAP and immunoassays of PAP are in progress.